Thanks for your reply. I would not have been surprised at all had I missed a line about "this" being the centre seam, but alas, there is no such line. Here's the pattern for the body piece:
BODY (MAKE 2)
* With 2 3/4mm (No 12/US2) needles cast on 3 sts. K 1 row. Cont in st st, inc one st at each end of next 2 rows and 2 foll alt rows. 11 sts.* Break off yarn. Work from * to *. P1 row across both sets of sts. 22 sts. Work 12 rows straight. Dec one st st each end of next row and 2 foll 3rd rows, then on foll alt row. 14 sts. P1 row. Cast off.
TO MAKE UP
Join centre seam on each body piece. Join body pieces together, leaving cast off edge open. Stuff and gather open edge, pull up and secure.
Actually, I didn't understand what "Dec one st st each end of next row and 2 foll 3rd rows" meant either (the 2 foll 3rd rows part), but figured it wouldn't make a huge difference in the way the piece looked.
I'm attaching a photo of my finished piece. Any help is much appreciated!